The reflected and radiated acoustic fields of a thick, composite plate of infinite extent, consisting of a substrate layer bonded at an interface to a coating layer, which has fluid on one side and which is unconstrained on the other side, are calculated theoretically. The reflected and radiated acoustic fields are related by means of a structural response function that completely characterizes the elastic behavior of the composite plate and of the fluid. The composite-plate acoustical problem is analyzed by extending the Timoshenko-Mindlin theory of thick homogeneous plates. The Timoshenko-Mindlin thick-plate equations are used to describe the flexural waves generated in each of the two layers of the composite plate, owing either to a plane acoustic wave in the fluid impinging on the plate or to a point-force excitation of the plate. Two ideal types of bonding between the coating and the substrate are considered: the 'welded' bond, for which contiguous plate elements on either side of the interface move in complete unison, and the 'perfectly slipping' bond, for which such plate elements move in unison normal to the interface but independently parallel to the interface. The analytic form of the structural response function of a composite plate shows that in general a thick bilaminar composite plate of the type considered cannot be modeled by a simple homogeneous thick plate with artificial or 'average' material constants. The analysis also shows that the bond between the substrate and the coating significantly affects the acoustic reflection and radiation characteristics of the composite plate. (Author)

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